1. Field of the Invention
The present invention relates to post-processing apparatuses that perform processing such as sorting and stapling for sheets of paper having an image formed thereon.
2. Description of Related Art
When a relatively large number of sheets of paper on which an image is formed by an image forming apparatus, such as a copier or a printer, need to be stapled (hereinafter “stapling”) or punched (hereinafter “punching”), a so-called “finisher”, which is a post-processing apparatus that automatically performs predetermined post-processing such as stapling or punching is useful.
FIG. 14 is a side sectional view showing an example of the structure of a conventional post-processing apparatus. In FIG. 14, a post-processing apparatus 100 is, for example, detachably connected to an image forming apparatus 101, such as a copier, a facsimile, or a printer, on a paper ejection side thereof. The post-processing apparatus 100 is provided with a processing tray 1 that can store a plurality of sheets of paper P. On the upper right of the processing tray 1 is provided a paper entrance 2. Near the paper entrance 2 is disposed a pair of receiving rollers 3. The sheets of paper having an image formed thereon are stapled, for example, as follows. The sheets of paper P sequentially ejected through a pair of ejection rollers 102 of the image forming apparatus 101 pass through the paper entrance 2 into the post-processing apparatus 100, are then conveyed to the processing tray 1 through the pair of receiving rollers 3 and a paper conveying path R, and are then temporarily stored in the processing tray 1.
The processing tray 1 is provided with a paper raising/lowering member 9 that can move up and down along a paper receiving surface. When the sheet of paper is being conveyed to the processing tray 1, the paper raising/lowering member 9 is made to wait in the lower part of the processing tray 1, whereby the leading edges of the sheets of paper P that have sequentially been conveyed through the pair of receiving rollers 3 to the processing tray 1 are supported by the paper raising/lowering member 9. Near the position where the paper raising/lowering member 9 is made to wait is provided a stapler 5. The stapler 5 staples a stack of paper whose leading edges are evened up by the paper raising/lowering member 9. The stapled stack of paper is conveyed to the upper part of the post-processing apparatus 100 by the paper raising/lowering member 9, and is then ejected through a pair of stack ejection rollers 6 into an eject tray 7.
With respect to the post-processing apparatus described above, the following problem arises. If the rear end of the sheet of paper conveyed to the processing tray 1 floats due to curling or static electricity, the leading edge of the next sheet of paper reaches under the sheet of paper that has already been placed on the processing tray 1, resulting in an undesirable change in the sequence of the stacked sheets of paper. Alternatively, the leading edge of the next sheet of paper bumps into the rear edge of the sheet of paper that has already been placed on the processing tray 1, resulting in the occurrence of a paper jam. This hinders the sheets of paper from being conveyed or stacked smoothly.
To avoid this problem, a method for preventing the sheets of paper from being unsuccessfully stacked due to floating of the rear end of the sheet of paper conveyed to the processing tray has been proposed. For example, JP-A-H11-292387 (hereinafter “Patent Document 1”) discloses a post-processing apparatus provided with a swingable paper pressing portion that presses the rear end of the sheet of paper placed on a processing tray against the tray. According to the method disclosed in Patent Document 1, when a sheet of paper is conveyed to the processing tray and the rear end thereof passes through a pair of receiving rollers, a pressing member moves from a retracted position (a home position) to a paper pressing position so as to press the rear end of the sheet of paper against the processing tray. When the stapled stack of paper is ejected into the eject tray, the pressing member moves to the paper pressing position so as to serve as a stack ejection guide.
To achieve a proper swinging motion of the paper pressing portion between the retracted position and the paper pressing position, the movement of the paper pressing portion has to be regulated by using a stopper member located outside the range in which the paper pressing portion moves. By doing so, however, the paper pressing portion collides with the stopper member, causing collision noise. In addition, there is a possibility that the paper pressing portion collided with the stopper member and bounded back may fail to move to a given position, interfering with the reception of the next sheet of paper.
On the other hand, JP-A-2006-16090 (hereinafter “Patent Document 2”) discloses a paper stacking mechanism that reduces the collision noise by reducing the speed of a swingable paper pressing plate by a gear mechanism, and enhances swing accuracy by minimizing the bound of the paper pressing plate. Certainly, with the mechanism disclosed in Patent Document 2, it is possible to stabilize a swinging motion of the paper pressing plate. However, since the paper pressing plate is retracted from the paper pressing position when the next sheet of paper is received, there is a possibility that the rear end of the stacked sheet of paper moves again away from the surface of the tray if it strongly curls or builds up static electricity. In addition, as is the case with Patent Document 1, since the paper pressing portion needs some time to swing back and forth, it is difficult to adopt this mechanism to a model that conveys a sheet of paper at high speed. It is for this reason that the above-described mechanism is not good enough to prevent the sheets of paper from being unsuccessfully stacked, and is susceptible to further improvement.